In the manufacture of tissue roll products, such as bath tissue and paper towels, uncreped throughdried products have gained wide acceptance with consumers. These products are characterized in part by their high bulk, three-dimensional texture and resilience. In the case of paper towels, exceptional bulk is provided by contoured throughdrying fabrics that impart high and wide wales or ridges that run in the machine direction of the product. In the case of bath tissues, the same technology is utilized, but the throughdrying fabrics employed impart a smaller scale topography to the product. While it would be desirable to use the same throughdrying fabric for both towels and bath tissue from the standpoint of manufacturing efficiency, using the more highly contoured towel throughdrying fabric for making bath tissue causes two significant problems.
First, the consumer preferred fiber basis weights and tensile strengths associated with bath tissue products are, for the most part, less than the basis weights and tensile strengths preferred for paper towels. Given the high contour of the fabrics used for paper towel products, the lower basis weights and tensile strengths used for bath tissue products cannot accommodate the substantial z-directional displacement of the web during wet molding and drying. As a result, the final product contains an unacceptable number of pinholes caused by the web being stretched to conform to the topography of the throughdrying fabric.
In addition, because bath tissue is desirably calendered to control caliper and soften and smoothen the product, the dried web undergoes widening as it is xe2x80x9cextrudedxe2x80x9d from the calender nip. This web widening is amplified as the bulk of the tissue base sheet is increased. This extrusion phenomenon creates inconsistencies during winding, which results in substantial waste and delay.
Therefore there is a need for a method of making highly contoured uncreped throughdried paper towels and bath tissue on the same tissue machine using the same throughdrying fabric.
It has now been discovered that highly textured bath tissue and paper towels having different basis weights can be made on the same tissue machine using a common throughdrying fabric. This provides manufacturing flexibility by eliminating the need to change throughdrying fabrics whenever switching from bath to towel manufacture or vice versa. It also simplifies fabric purchasing and inventorying.
In one aspect, the invention resides in a papermaking fabric having a textured sheet contacting surface comprising substantially continuous machine-direction ridges separated by valleys, wherein the height of the ridges is from about 0.5 to about 3.5 millimeters, the width of the ridges is about 0.3 centimeter or greater, and the frequency of occurrence of the ridges in the cross-machine direction of the fabric is from about 0.2 to about 3 per centimeter. The fabric can be woven or nonwoven, or a combination of a woven substrate with an extruded sculpture layer providing the ridges.
In another aspect, the invention resides in a continuous method of making bath tissue and paper towels on the same papermaking machine comprising: (a) forming a tissue web having a first basis weight; (b) transferring the tissue web to a throughdrying fabric having substantially continuous machine-direction ridges separated by valleys, wherein the height of the ridges is from about 0.5 to about 3.5 millimeters, the width of the ridges is about 0.3 centimeter or greater and the frequency of the ridges in the cross-machine direction is from about 0.2 to about 3 per centimeter; (c) throughdrying the tissue web; (d) winding the tissue web into a parent roll; (e) converting the parent roll into bath tissue; (f) forming a tissue web having a second basis weight which is greater than the first basis weight; (g) transferring the web to the same throughdrying fabric of step (b); (h) throughdrying the web; (i) winding the dried web into a parent roll; and (j) converting the parent roll into paper toweling.
In another aspect, the invention resides in a tissue sheet having Wide Wales, a basis weight of from about 10 to about 35 grams per square meter (gsm) and one or more of the following pinhole-related indexes: a Pinhole Coverage Index of about 0.25 or less, a Pinhole Count Index of about 65 or less and a Pinhole Size Index of about 600 or less.
In another aspect, the invention resides in a tissue sheet having Wide Wales and a geometric mean tensile strength of from about 500 to about 1200 grams per 7.62 centimeters, a basis weight of from about 10 to about 45 gsm and one or more of the following pinhole-related indexes: a Pinhole Coverage Index of about 0.25 or less, a Pinhole Count Index of about 65 or less and a Pinhole Size Index of about 600 or less.
As used herein, xe2x80x9cWide Walesxe2x80x9d are a series of parallel ridges on the surface of a tissue sheet which are separated by the lowest areas of the sheet (valleys). The Wide Wales are oriented substantially in the machine direction (MD) of the tissue sheet and impart a surface appearance similar to that of corduroy fabrics. The peaks of the ridges can be relatively flat and the sides of the ridges can be relatively steep. The width of a Wide Wale can be from about 0.3 to about 3.8 centimeters, more specifically from about 0.3 to about 2.0 centimeters, more specifically from about 0.3 to about 1.5 centimeters, more specifically from about 0.3 to about 1.0 centimeter, and still more specifically from about 0.3 to about 0.5 centimeter. The height of a Wide Wale, as measured from the highest point on the ridge to the lowest point on the same side of the sheet between the ridge in question and an adjacent ridge, can be from about 0.5 to about 3.5 millimeters, more specifically from about 0.6 to about 2.0 millimeters, more specifically from about 1.0 to about 2.0 millimeters, more specifically from about 1.0 to about 1.5 millimeters, and still more specifically from about 0.75 to about 1.0 millimeters. The frequency of the occurrence of Wide Wales in the cross-machine direction (CD) of the sheet can be about 0.2 to about 3 per centimeter, more specifically from about 0.2 to about 2 per centimeter, still more specifically from about 1.8 to about 2.3 per centimeter. All of the foregoing dimensions substantially correspond to the dimensions of the ridges and their spacing in throughdrying fabrics from which the tissue sheets are made.
The basis weight of the tissue sheets of this invention can be from about 10 to about 45 gsm, more specifically from about 10 to about 35 gsm, still more specifically from about 20 to about 35 gsm, more specifically from about 20 to about 30 gsm and still more specifically from about 30 to about 35 gsm.
The geometric mean tensile strength (GMT) of the tissue sheets of this invention can be about 1200 grams or less per 7.62 centimeters (hereinafter designated simply as xe2x80x9cgramsxe2x80x9d), more specifically from about 500 to about 1200 grams, still more specifically from about 500 to about 1100 grams, still more specifically from about 800 to about 1000 grams. The GMT is the square root of the product of the MD tensile strength and the CD tensile strength. Tensile strengths are measured using a crosshead speed of 254 millimeters per minute, a full scale load of 4540 grams, a jaw span (gauge length) of 50.8 millimeters and a specimen width of 762 millimeters. A suitable method is disclosed in U.S. Pat. No. 5,656,132 issued Aug. 12, 1997 to Farrington et al., which is herein incorporated by reference.
The ratio of the geometric mean modulus (GMM) to the GMT for tissue sheets of this invention can be about 5 kilometers or less per kilogram, more specifically from about 4 to about 5 kilometers per kilogram. (The GMM is the square root of the product of the MD modulus and the CD modulus.) The xe2x80x9cCaliperxe2x80x9d of the products of this invention can be from about 700 to about 1500 microns, more specifically from about 700 to about 1300 microns, and still more specifically from about 750 to about 1100 microns. Caliper is the thickness of a single sheet, but measured as the thickness of a stack of ten sheets and dividing the ten sheet thickness by ten, where each sheet within the stack is placed with the same side up. Caliper is expressed in microns. It is measured using a micrometer having an anvil diameter of 103.2 millimeters and an anvil pressure of 220 grams per square inch (3.3 gram kilopascals. A suitable test method is described in U.S. Pat. No. 5,656,132 issued Aug. 12, 1997 to Farrington et al., previously incorporated by reference. Uncreped throughdried tissue sheets of this invention have a substantially uniform density.
The tissue sheets of this invention can be layered or non-layered (blended). Layered sheets can have two, three or more layers. For tissue sheets that will be converted into a single ply product, it can be advantageous to have three layers with the outer layers containing primarily hardwood fibers and the inner layer containing primarily softwood fibers.
As used herein, the xe2x80x9cPinhole Coverage Indexxe2x80x9d, the xe2x80x9cPinhole Count Indexxe2x80x9d and the xe2x80x9cPinhole Size Indexxe2x80x9d are determined by an optical test method which, in conjunction with image processing algorithms, isolates pinholes and provides coverage (percent area), count (number per 100 square centimeters) and size (equivalent circular diameter) for pinholes within the tissue sheet. The method uses a fluorescent ring illuminator to provide omni-directionality, high intensity and appropriate wavelength for incident-light detection of pinholes. Further, the method uses an image processing sequence of multiple sequential xe2x80x9copeningsxe2x80x9d and xe2x80x9cclosingsxe2x80x9d to cluster appropriate sub-holes into a pinhole.
More specifically, a tissue sheet sample is placed on an auto-macrostage, resting on a Kreonite Mobil Studio macroviewer, under a 50 mm lens attached to a chalnicon scanner (TV camera). The sample is imaged over a black background and covered by a xe2x85x9 inch thick glass plate. The key lighting is provided by a 6 inch Aristo Ring illuminator with a xe2x80x9ccoolxe2x80x9d white bulb, providing incident omni-directional illumination. The variable neutral density filters (VNDFs) are used beforehand to xe2x80x9cget closexe2x80x9d to the proper white level response, with the auto-sensitivity function used during program execution then taking over to provide a xe2x80x9cwhite levelxe2x80x9d=1.00. The autostage is moved to 25 adjacent field locations, each having a field size (live frame) of 15 mm. by 13 mm. The particular equipment to be used is: a Quantimet 970 Image Analysis System or equivalent; IDC HM1212 auto-macrostage; 50 mm EI-Nikkor lens at f/5.6; variable neutral density filters (VNDFs); 20 mm. extension tube; Aristo Microlite M-II 6-inch fluorescent ring illuminator with cool white bulb; black photo-drape background; xe2x85x9 inch covering plate glass; and a chalnicon scanner. Shading correction was set manually before program execution on high basis weight calendered computer paper.
The software routine to process the image is as follows:
The xe2x80x9cPinhole Coverage Indexxe2x80x9d is the arithmetic mean percent area of the sample surface area, viewed from above, which is covered or occupied by pinholes. It is represented by PERCAREA in the foregoing software program. For purposes of this invention, the Pinhole Coverage Index can be about 0.25 or less, more specifically about 0.20 or less, more specifically about 0.15 or less, and still more specifically from about 0.05 to about 0.15.
The xe2x80x9cPinhole Count Indexxe2x80x9d is the number of pinholes per 100 square centimeters that have an equivalent circular diameter (ECD) greater than 400 microns. It is represented by the total FEATURE COUNT in the histogram output from the foregoing software program, which is then manually divided by the TOTAL AREA SCANNED in the foregoing software program. For purposes of this invention, the Pinhole Count Index can be about 65 or less, more specifically about 60 or less, more specifically about 50 or less, more specifically about 40 or less, still more specifically from about 5 to about 50, and still more specifically from about 5 to about 40.
The xe2x80x9cPinhole Size Indexxe2x80x9d is the mean equivalent circular diameter (ECD) for all pinholes having an ECD greater than 400 microns. It is represented by CALC in the foregoing software program. For purposes of this invention, the Pinhole Size Index can be about 600 or less, more specifically about 500 or less, more specifically from about 400 to about 600, still more specifically from about 450 to about 550.